Hydrogen is a promising alternative fuel source to petroleum-based fuels currently used for transportation. It can be used either in a fuel cell to power an electric motor or burned directly in an internal combustion engine, with the only waste byproduct being water. Hydrogen does not exist naturally in significant quantities, so it must be derived from other sources such as natural gas or water. With the use of alternative energy sources to produce hydrogen, its widespread use would lead to a reduction of greenhouse gases created and help alleviate the need for petroleum.
|Table 1: A comparison of 2015 fuel storage system goals in 2005 and 2009, along with system performance of an average commercial vehicle.|
Hydrogen has a naturally low energy density by volume compared to gasoline. A theoretical maximum energy density can be calculated by assuming the density of liquid hydrogen is the highest attainable density. The density of liquid hydrogen is 0.07 g/cm3, which corresponds to an energy density of 2.8 kWh/L assuming perfect combustion.  The energy density of gasoline is about three times higher at 10 kWh/L.
Hydrogen's low energy density poses a problem for designing onboard hydrogen storage for vehicles - the fuel system should ideally be approximately the same size and weight as one in a gasoline-powered car. Developing this hydrogen storage capacity has been one of the goals of the FreedomCAR and Fuel Partnership (FCFP), a partnership between DOE, The U.S. Council for Automotive Research, and energy companies.
The FCFP laid out specific technical targets for hydrogen storage capability in 2005, the most significant factors being system gravimetric capacity, system volumetric capacity, and storage system cost. Their long term goal was to have a competitive hydrogen storage system by 2015. The target performance was determined to be that of an average commercial vehicle (minivan, light truck, etc). This average vehicle holds 19.8 gallons of fuel, has an efficiency of 18.7 mpg, and has a fuel system weighing 74 kg.  This equates to a specific energy of 8.9 kWh/kg. Assuming a fuel economy gain of 3x, the goal of 3 kWh/kg was determined. The average commercial vehicle's energy density is 6.15 kWh/L, which suggests a target of 2 kWh/L. However, gas tanks are designed to conform to the free space in a car and hydrogen tanks do not have that luxury since they are likely to be pressurized, so a higher target of 2.7 kWh/L was set. Finally, the fuel system of the average commercial vehicle costs $0.41/kWh, so after accounting for the fuel economy boost, the goal was $2/kWh. A summary of these goals can be seen in Table 1.
The assumption of a fuel economy gain of 3x is quite absurd. When switching to hydrogen, the fuel economy is likely to decrease by a factor 3 due to hydrogen's lower energy density. By fudging their calculations this way, the FCFP effectively removes an order of magnitude from their goals for hydrogen storage perfomance. Even with this fudge factor, the volumetric capacity goal of 2.7 kWh/L is very close to the theoretical limit of 2.8 kWh/L, making it very hard to achieve.
The FCFP likely found that their target performance would not be reachable by 2015, which is why they revised their goals in 2009.  Under these new goals, two technologies have been identified that are likely to meet the targets: MOF-177 and cryo-compressed hydrogen. [4,5] MOF-177 is a metal-organic framework that has a very large surface area to capture hydrogen molecules and increase the amount of hydrogen stored in a tank at high pressures. Cryo-compressed hydrogen already meets the 2015 target in gravimetric and volumetric capacity. In this scheme, gaseous hydrogen just above the boiling point (20.3 K) is used to fill an insulated tank, allowing high energy densities. The gas in the tank will eventually heat up and expand, creating hundreds of atmospheres of pressure. Burning fuel by driving the vehicle keeps this pressure in check.
From the current state of hydrogen storage technology, it is apparent that hydrogen-powered vehicles will not be competitive with gasoline vehicles in the short term. The newest storage technologies still have significant hurdles to overcome, but they may become competitive in the future.
© Alex Contryman. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
 N. Wiberg, A. F. Holleman and E. Wiberg, Inorganic Chemistry (Academic Press, 2001).
 "Hydrogen Storage Technologies Roadmap," FreedomCAR and Fuel Partnership, November 2005.
 "Targets for Onboard Hydrogen Storage Systems for Light-Duty Vehicles," Office of Energy Efficiency and Renewable Energy, U.S. Department of Energy, September 2009.
 H. Furukawa, M. A. Miller and O. M. Yaghi, "Independent Verification of the Saturation Hydrogen Uptake in MOF-177 and Establishment of a Benchmark for Hydrogen Adsorption in Metal-Organic Frameworks," J. Mat. Chem. 17, 3197 (2007).
 R. K. Ahluwalia et al., "Technical Assessment of Cryo-Compressed Hydrogen Storage Tank Systems for Automotive Applications," Intl. J. Hydrogen Energy 35, 4171 (2010).